Direct radio frequency communication systems and wide bandwidth instrumentation systems are two important sources of motivation for designing extremely high speed analog-to-digital converters (ADCs). In one example, such systems may require an ADC which can provide 12-bit resolution running at 10 Gigasamples/second. Certain process nodes can help make circuits in such high speed ADCs run faster, but can pose new challenges for circuit designers. For instance, some process nodes may require lower voltage designs. Limits on power consumption or general preference for lowering power consumption may also require lower voltage designs. At the same time, circuit designers aim to design circuits which can perform well.
One important part of an ADC are the amplifiers, which are used to amplify signals. One may find amplifiers connecting stages of a pipeline ADC, or other ADC architectures. The amplifier is an important part of the signal chain in an ADC, and many other analog circuits. Performance of the amplifier can directly affect many performance metrics of the overall system.
To achieve certain benefits such as even order harmonics cancellations, differential or balanced circuits are often used to implement amplifiers. For differential circuits, including differential buffers, differential amplifiers, and push pull differential amplifiers, common mode of a differential voltage may need to be adjusted for optimal performance or requirements of the overall circuit. Common mode of a differential voltage is a common voltage that appears in both signals, i.e., both the + and − parts of the differential voltage. Common mode adjustments or controls may be particularly useful when multiple stages of differential circuits are cascaded one after another, and one stage may impose common mode requirements on another stage. Common mode feedback (CMFB) circuits are used for adjusting/controlling common mode of differential voltages. Specifically, a CMFB circuit (sometimes referred to as a CMFB loop) can be are added to differential circuits to adjust input common mode and/or output common mode of the differential circuits.